1) Field of Invention
This invention relates to a method and an apparatus for the on-line determination of the degree of fiber surface development of pulp during stock preparation by refining or beating (the term `refining` henceforth will be used to denote both `refining` and `beating`).
2) Description of the Prior Art
Refining is one of the most important stages of stock preparation in paper making. It provides the finished paper with such specific properties as strength, tear, bulk, bonding, rigidity, opacity, formation etc. It also influences the drainage characteristics of the pulp stock and consequently affects the sheet-making operation on the paper machine.
Refining is applied to the production of both mechanical and chemical pulps but in rather different ways. In mechanical pulping, refining is an integral part of the pulping process since it is employed to convert the wood directly into fibers. In chemical pulping, refining is a secondary operation. The fibers are first separated by cooking the wood chips with chemicals and are then subsequently refined. The action of refining produces increased external and internal fibrillation, and fiber shortening. It is these phenomena which lead to the improvement of the finished paper properties.
Historically, the degree of refining of both mechanical and chemical pulps has been monitored by means of freeness measurements applied to samples withdrawn from the process. The freeness test is essentially a drainage test; it is normally performed with the Canadian Standard Freeness (or the Schopper-Riegler) tester. This test was originally developed on an empirical basis, and although it has therefore been frequently criticized for this reason, it has been maintained as a standard over many years because it is simple to operate and provides a measurement rapidly.
In recent times, a number of on-line drainage devices have been developed to replace the off-line freshness test in order to provide an automated measurement and hence improved process control. Such instruments as the Bolton-Emmerson Drainac, the Innomatic, the Koei Newfreenester, the Kajaani PDA (Pulp Drainage Analyzer), the Bonnier DRT (Drainage Rate Transmitter), and the Sunds PQM (Pulp Quality Monitor) have been introduced. These devices, while providing improved process control, do not generally produce measurements which directly represent freeness. Furthermore, in the operation of these devices, there is invariably a need for the measurement or control of the consistency of the stock.
Since the empirical character of the freeness and drainage measurements make them unsatisfactory as standards, many researchers have attempted to provide an alternative, more comprehensive characterization of pulp in terms of other, fundamental properties. For example, Clark (Tappi 45(8): 628 (1962) proposed fiber length, coarseness, cohesiveness, intrinsic strength and wet compactibility; Bridge and Hammer (Paper Tech Ind 18(12): 37 (1977) suggested fiber length, coarseness, intrinsic strength, fiber length distribution, specific surface area, and area bonding potential. Forgacs (PPMC 64(C): T89 (1963), in his attempts to characterize groundwood, proposed two properties alone, L factor, related to fiber length, and S factor, related to the specific surface of the 48-100 mesh screen fraction. Additional properties suggested by other researchers have included other forms of fiber length, fiber size distribution, specific volume, cell wall thickness, flexibility, compressibility, fiber/fines ratio, shive content and water retention among others. Clearly, many of these are inter-related.
To reduce the number of the preceding fiber properties to a minimum that may be considered as the basic parameters, there are three which have been identified as occurring the most frequently, by Casey (Pulp and Paper, 3rd. Ed, Vol. 2, p. 839 (1980)) for example, and which appear best to satisfy the criteria.
These are:
fiber length; in refining, the fiber length is subject to reduction by refiner cutting action PA1 fiber specific surface; in refining, the specific surface is modified by external fibrillation or fiber splitting PA1 flexibility; in refining, the flexibility is modified by internal fibrillation or bruising.
The manner in which, and by what amount, each of these properties contribute to the freeness is difficult to delineate specifically since the results from using different pulp types and different refining processes are not the same. However, the measurement of the specific surface area, which would indicate the increase in fiber surface development during refining, would provide the most direct index of the quality of refining. The measurement of the external specific surface area of pulp fibers has been performed by diverse techniques which include microscopic, fiber silvering and nitrogen adsorption measurements. However, it has been more generally obtained from drainage measurements of the type described by Ingmanson (Tappi 35(10): 439(1952), Parker and Mih (Tappi 47(5): 254(1964), Kerstin Olander, (Tappi 81(1991), Hammer (Paper Tech 15(5): 263(1974), and by Robertson and Mason (PPMC 50(13): 1 (1949). The permeability method of Robertson and Mason provides the means that is most commonly used; this approach has been implemented commercially through the introduction of the Pulmac Permeability Tester.
In addition to drainage methods, optical methods have also been proposed for the measurement of specific surface or similar surface development characterizations.
Mason (Tappi 33(8): 403(1950)), described a laboratory technique in which the transmission of light through a pulp suspension was attenuated by changes in fiber specific surface and other pulp properties. This approach required a measure of consistency.
Silvy and Pascal (ATIP (22): 205(1968)) measured signals from angularly diffracted light from a beam transmitted through a pulp suspension; the ratio of these signals provided an index of the degree of refining.
Pettersson, Fladda and Lundquist (patent Sweden 7706320 (Dec. 27, 1987)) described a method for the measurement of the size distribution of solids in a flowing suspension. This was effected by the detection of light transmitted through the suspension, and by a combination of the separate AC and DC components of the signal.
Pettersson and Karlsson (patent U.S. Pat. No. 4,529,309 (Jul. 16, 1985)) determined the average radius and/or the average length of particles carried by a flowing medium. In this case, signals were obtained from two detectors measuring radiation through a particle suspension, and the AC component from one was combined with the DC component of the other.
Forgacs and Karnis (Canadian Patent 938128 Dec. 11, 1973)), Forgacs and Karnis (U.S. Pat. 3,802,964 (Apr. 9, 1974)), Karnis and Wood (U.S. Pat. No. 4,135,389 Jan. 23, 1979)), Karnis and Shallhorn (U.S. Pat. No. 4,276,119 (Jun. 30, 1981)), Karnis (Canadian patent 1,123,626 (May 18, 1982)), and Karnis and Shallhorn U.S. Pat. No. 4,441,960 (Apr. 10, 1984) determined the specific surface of pulp fibers from turbidity measurements at known consistencies.
Simms and Madson (U.S. Pat. No. 4,159,639 (Jul. 3, 1979)) described an optical method for measuring the degree of refining by monitoring the rate of descent of a pulp-water interface of a pulp suspension at a selected consistency.
Shimuzu, Usuda and Kadoya (Japan Tappi 35(7)): 609(1981)), compared the intensity of signals of light transmitted directly through a pulp suspension and light scattered at an angle to the forward axis, and related a combined signal to the degree of beating of the pulp.
Fedko, Dorf and Slavutskii (patent SU 1075123 (Feb. 23, 1984)) described the measurement of specific surface by light scattering of particles as they settle in a column of liquid.
Unger, Heinemann, Trankner and Strassberg in patent DD 218463 (Feb. 6, 1985) and the paper, "Comparative examination of different methods for determining specific surface of pulp fibers in suspension", Zellstoff Papier 32, no 4: 154:157 (July/August 1983)) described an invention which uses turbidity measurements to determine light scattering and hence specific surface.
Bott (U.S. Pat. No. 4,676,641 (Jun. 30, 1987)), described a goniometric technique for measuring light scattering, principally of very small particles in suspension, and related these measurements to the particle size distribution.
Most of these methods require an accurate consistency measurement of the pulp stock.
Meyn, Landmark and Aagedal (J. Institute of Measurement and Control 1(9); T165(1968) described an optical method for measuring pulp consistency. This employs a combination of signals from polarized light beams transmitted through pulp stock, one beam with its plane of polarization in line with that of the incident beam, the second with its plane of polarization at ninety degrees.
Two commercial instruments, the EUR-Control Lowcon and the Kajaani LC-100 were designed using this principle of operation. Simms and Madson (U.S. Pat. No. 4,171,916 (Oct. 23, 1979)) described a method which is also based on the same principle of operation.
Saltzman (U.S. Pat. No. 3,283,644 (Nov. 8, 1966)) at an earlier date described the same method applied to the measurement of concentrations of solids in suspensions.
Leschonski (patent DE 3105752 (Jan 17, 1981)) measured forward scattered light by light extinction and gamma (or similar) radiation to determine concentration.